1. Field of the Invention
The invention relates to a structure with bumps, particularly to a structure of bumps forming on an under bump metallurgy layer and a method for making the same.
2. Description of the Related Art
FIGS. 1A to 1F show a conventional method for forming bumps on the under bump metallurgy layer. Firstly, referring to FIG. 1A, a wafer 10 is provided. The wafer 10 has an active surface 101, and the active surface 101 has a plurality of solder pads 103. A protection layer 102 covers the active surface 101. The protection layer 102 has a plurality of openings 104, wherein the openings 104 are correspondingly on the solder pads 103 so as to expose parts of the solder pads 103. Referring to FIG. 1B, an under bump metallurgy layer 11 (UBM layer) is formed on the protection layer 102 and the solder pads 103. The under bump metallurgy layer 11 comprises an adhesion layer 111, a stress buffer layer 112 and a wetting layer 113. The solder pads 103 electrically connect to the under bump metallurgy layer 11.
A first photo resist 12 is formed on the first under bump metallurgy layer 111. The first photo resist 12 has a plurality of openings 121, and the openings 121 are formed on the under bump metallurgy layer 11, corresponding to the solder pads 103. Referring to FIG. 1C, a solder paste 13 is filled into the openings 121 of the first photo resist 12. Referring to FIG. 1D, the first photo resist 12 is then removed. Referring to FIG. 1E, the adhesion layer 111 is etched so that the adhesion layer 111, the stress buffer layer 112 and the wetting layer 113 are of the same width. Referring to FIG. 1F, the solder paste 13 is heated to form a bump 14.
According to the conventional method for forming bumps on the under bump metallurgy layer, since the size of the adhesion layer 111 is the same as those of the stress buffer layer 112 and the wetting layer 113, the molten solder paste 13 flows down to the lateral of stress buffer layer 112 during a reflow process. Therefore, the problem of stress concentration will happen so that the bump 14 is unstable and peels easily due to the IMC (Intermetallic Compounds) formed between the bump 14 and stress buffer layer 112.
Consequently, there is an existing need for providing a structure of bumps forming on the under bump metallurgy layer and a method for making the same to solve the above-mentioned problems.